Field of Endeavor: This invention relates to optical devices for ophthalmic applications.
The invention in this application is concerned with prisms that are thinner, lighter and have lower aberrations than the currently available ophthalmic prisms for vision applications. In optical systems, when deflection of light is desired, either mirrors or prisms are commonly used. Since mirrors are not transparent and cannot be put in the path of the beam to deflect it, they present a problem in several optical systems. Prisms are better suited for such systems.
Since most vision related devices use a direct viewing system, prisms are extensively used in many ophthalmic applications. Such applications are well known and documented in many books, for example “Clinical Uses of Prisms: A Spectrum of Applications”, edited by Susan Cotter, published by Mosby-Year Book, Inc. (1995). Prisms are used in devices such as spectacles to either correct visual disorders or to improve the ability of the patient to perform certain visual tasks. Prisms are particularly useful for people with partial vision or low vision. Many applications of prisms for low vision have been described in the literature and have been summarized in books such as “Primary Low Vision Care”, by Rodney Nowakowski, published by Appleton and Lange (1994). Prisms are also used in the diagnosis of many visual disorders. Due to such broad range of applications, a set of prisms with powers up to 50 prism diopters is a standard equipment in the office of practically every ophthalmologist and optometrist. Prism diopter (p.d.) is defined as (the tangent of the angle of deflection)×100.
Several ophthalmic devices use prisms to shift or expand the visual field. For example, in U.S. Pat. 4,779,972, a visual field awareness system is described, where spectacles with appropriately placed prisms are used to increase the visual field of hemianopic patients. Inwave Optics, Inc., markets spectacles with multiple prisms called Field Expanding Channel Lens for patients with peripheral visual field loss. New ophthalmic devices that use prisms to correct or improve the vision continue to emerge. For example, U.S. Pat. No. 6,318,857, shows variable power spectacles which utilize a series of prisms. Similariy in U.S. Pat. No. 5,969,790, a multi prism system is used for image enhancement. Researchers at Harvard Medical School's Schepens Eye Research Institute have recently developed new devices to expand the visual field of visually impaired brain damaged patients (cf. E. Peli et al., Optometry and Vision Science, September 2000), which make use of multiple prisms. Currently several visual disorders are corrected by using ophthalmic refractive prismatic spectacles. High magnification prismatic reading spectacles and spectacles used to correct diplopia, hemianopia and strabismus are some examples.
Although there are numerous possible applications for prisms in ophthalmic devices, the problem is that for most applications, the standard ophthalmic refractive prisms can only be used for low prismatic powers of up to 10-12 p.d. The reason is that for high prism diopters, such prisms become thick and heavy. The use of such prisms in spectacles and other vision related devices (for example spectacle mounted telescopes) becomes particularly difficult. The reason is that spectacles with heavy prisms are uncomfortable to wear and spectacles with thick prisms are impractical to use because they touch the user's eyelashes. For high prism diopters, standard ophthalmic prisms also display poor optical quality due to higher chromatic and monochromatic aberrations. For ophthalmic devices, where high prismatic powers are needed, Fresnel prisms are usually used because they are thin. Fresnel prisms overcome the thickness problem, but present other serious problems, such as a significant reduction in visual acuity and contrast sensitivity (cf. Woo G. C., Campbell F. W. and Ing B in Ophthalmic Physiol. Opt. 6, pp 415-418 (1986)). High refractive index materials can be used to reduce the thickness of the refractive ophthalmic prisms, but such high index materials display higher dispersion. This deteriorates their optical quality drastically, making high index material prisms unacceptable for most vision applications.
Prior to the instant invention, refractive ophthalmic prisms and Fresnel prisms were the only two choices available for correcting vision conditions and these are both impractical or unsatisfactory for high diopter (over 15 prism diopter) prisms. However, prism diopters (p.d.) as high as 40 are needed to correct some visual conditions and prisms up to 50 p.d. are needed for diagnostic purposes.